1. Field of the Invention
The present invention relates to an excimer lamp as an ultraviolet light source that is mainly used for industry. Especially, it relates to a structure of an excimer lamp that radiates an excimer ray by either dielectric barrier discharge or capacitively-coupled high-frequency discharge.
2. Description of the Related Art
As an excimer lamp for industry, a xenon lamp having a wavelength of 172 nm is known, and is used for cleaning a substrate. An excimer lamp with a dual-cylinder tubular structure is generally used, and a light-emitting part of the lamp is formed by two co-axially cylindrical long tubes that extend along the axial direction.
For example, an excimer lamp enclosing xenon gas is used for the dry-cleaning of an LCD panel substrate, as described in Patent Document No. 1 (Japanese Patent No. 3170952). In this case, a substrate to be illuminated moves on a conveyor belt at a constant speed, and a lamp is arranged slightly above the substrate and in a direction perpendicular to a conveyer-flow direction. Since the substrate moves at a constant speed while the lamp illuminates the entire width of the substrate, the cleaning process for the whole of the substrate can be carried out uniformly. Also, in the semiconductor manufacturing field, there are many cases in which a surface process for a semiconductor wafer surface, such as a surface coating, surface modification, etc., is carried out by using an ultraviolet ray in each manufacturing step. An ultraviolet ray, such as an excimer ray having a wavelength of 172 nm based on xenon gas, or an excimer ray having a wavelength of 222 nm based on krypton and chlorine gases, is used in many of the cases.
In the dual-cylinder tubular type of excimer lamp using dielectric barrier discharge as described in the above Document 1, one of the electrodes is provided on an inner surface of an inner-side tube, and the other electrode is provided on an outer surface of an outer-side tube. The dielectric barrier discharge occurs in a discharge space between the inner-side tube and the outer-side tube by applying a high-frequency voltage of several kilo-volts between the electrodes. At this time, there is a possibility of an electrical breakdown and a creeping discharge along a discharge chamber surface, because of the application of several kilo-volts between the electrodes. It is necessary to create a sufficient amount of distance between both edge portions of the discharge chamber and the electrode edge portions or to add an electrical insulator to the discharge chamber edge portions in order to prevent the creeping discharge. However, such a construction cannot realize a compact and simple luminous unit since the conventional excimer lamp with the dual-cylinder tube structure is necessarily large.
On the other hand, as described in the Patent Document 2 (JP05-090803U), a fluorescent lamp (an outer-electrode type fluorescent lamp) in which electrodes are arranged on opposite sides of the exterior or outer surface of a single-cylinder tubular type discharge chamber is also known. In Document 2, the electrodes are covered with a coating layer consisting of a heat-proof material such as a glass bulb or ceramics, in order to prevent a creeping discharge and to enhance safety during the use of the lamp.
Even if the single-cylinder tubular type fluorescent lamp described in Document 2, which allows the diameter to be narrow, is used, a creeping discharge can still occur when a high voltage is applied to the electrodes. Since the single-cylinder tubular type of fluorescent lamp has a structure in which band-shaped electrodes are provided on an outer surface of a tube along the axis of a tubular discharge chamber, a design that lengthens an interval between the electrodes arranged along the outer surface can not be adopted. Therefore, it is necessary to cover the discharge chamber and the electrodes with an insulation material in order to prevent a creeping discharge.
To emit an excimer ray having high-radiation output characteristics, a gas pressure should be set at a high pressure (and especially an applied voltage should be set to a high voltage), however, a measure or step that only covers the electrodes with an insulation material will not cause reliability to improve. In the fluorescent lamp described in Document 2, since the pressure of a sealed gas may be set to a low level, it is not necessary to strengthen the resistance to insulation very much, even though the amount of excimer-molecules produced during a discharge is minimal and emitted light is weak. On the other hand, in an excimer lamp that applies high voltage to obtain high output of emitted light, there is a possibility of electric breakdown occurring through a slight clearance between the discharge chamber and the coating layer, though an electrode coating layer consisting of glass is heated and tightly attached to the electrodes. For example, when an aluminum foil is used as an electrode, it is difficult to coat the electrode in accordance with the electrode-shape without a clearance since the temperature of the heated glass cannot be increased sufficiently due to the low melting point of aluminum. Also, if the thermal expansion coefficient is different between the discharge chamber and the coating layer, stress occurs due to thermal hysteresis based on a blinking of the lamp, so that there is a danger that slight cracks leading to an electric breakdown may gradually occur in a boundary surface. Also, when laminating or covering by methods such as thermal spraying of glass materials, bubbling and cracking occur, which creates the danger of an electric breakdown occurring due to the bubbling and cracking. In this way, in the excimer lamp using the conventional single-cylinder tubular discharge chamber, a sufficient high voltage can not be applied, and a lamp with only low-radiation output characteristics is realized.